A reusable and versatile device/system for mixing liquid drugs or solutions, before their administration into the human body. Mixing is carried out inside an entirely closed circuit, avoiding any contacts between the phases to be mixed and the external atmosphere. Closure of the circuit is realized partly by integral and indissoluble (permanent) connections and partly by reversible connections, such as vascular accesses, unidirectional or bidirectional check valves (NRV), or any other device which allows the creation of a closed circuit even after use, both to the system and the connected accessories, thus minimizing the risks of possible contaminations and/or infections to the patients.